As compared to conventional cathode-ray tubes (CRTs) primarily used for realizing moving images, LCDs (Liquid Crystal Displays) have a drawback, so-called motion blur, which is the blurring of outline of a moving portion perceived by a viewer when displaying a moving image. It is pointed out that this motion blur arises from the LCD display mode itself (see, e.g., Specification of Japanese Patent No. 3295437; “Ishiguro Hidekazu and Kurita Taiichiro, “Consideration on Motion Picture Quality of the Hold Type Display with an octuple-rate CRT”, IEICE Technical Report, Institute of Electronics, Information and Communication Engineers, EID96-4 (1996-06), p. 19-26”).
Since fluorescent material is scanned by an electron beam to cause emission of light for display in CRTs, the light emission of pixels is basically impulse-like although slight afterglow of the fluorescent material exists. This is called an impulse display mode. On the other hand, in the case of LCDs, an electric charge is accumulated by applying an electric field to liquid crystal and is retained at a relatively high rate until the next time the electric field is applied. Especially, in the case of the TFT mode, since a TFT switch is provided for each dot composing a pixel and each pixel normally has an auxiliary capacity, the ability to retain the accumulated charge is extremely high. Therefore, the light emission is continued until the pixels are rewritten by the application of the electric field based on the image information of the next frame or field (hereinafter, represented by the frame). This is called a hold display mode.
Since the impulse response of the image displaying light has a temporal spread in the above hold display mode, spatial frequency characteristics deteriorate along with temporal frequency characteristics, resulting in the motion blur. Since the human eye can smoothly follow a moving object, if the light emission time is long as in the case of the hold type, the movement of image seems jerky and unnatural due to the time integration effect.
To improve the motion blur in the above hold display mode, a frame rate (number of frames) is converted by interpolating an image between frames in a known technology. This technology is called FRC (Frame Rate Converter) and is put to practical use in liquid crystal displaying devices, etc.
Conventionally known methods of converting the frame rate include various techniques such as simply repeating read-out of the same frame for a plurality of times and frame interpolation using linear interpolation between frames (see, e.g., Yamauchi Tatsuro, “TV Standards Conversion”, Journal of the Institute of Television Engineers of Japan, Vol. 45, No. 12, pp. 1534-1543 (1991)). However, in the case of the frame interpolation processing using the linear interpolation, unnaturalness of motion (jerkiness, judder) is generated due to the frame rate conversion, and the motion blur disturbance due to the above hold display mode cannot sufficiently be improved, resulting in inadequate image quality.
To eliminate effects of the jerkiness, etc., and improve quality of moving images, a motion compensation processing using motion vectors is proposed. Since a moving image itself is captured to compensate the image movement in this motion compensation processing, highly natural moving images may be acquired without deteriorating the resolution and generating the jerkiness. Since interpolation image signals are generated with motion compensation, the motion blur disturbance due to the above hold display mode may sufficiently be improved.
Above Specification of Japanese Patent No. 3295437 discloses a technology of motion-adaptively generating interpolation frames to increase a frame frequency of a display image for improving deterioration of spatial frequency characteristics causing the motion blur. In this case, at least one interpolation image signal interpolated between frames of a display image is motion-adaptively created from the previous and subsequent frames, and the created interpolation image signals are interpolated between the frames and are sequentially displayed.
FIG. 1 is a block diagram of a schematic configuration of an FRC drive display circuit in a conventional liquid crystal displaying device and, in FIG. 1, the FRC drive display circuit includes an FRC portion 100 that converts the number of frames of the input image signal by interpolating the image signals to which the motion compensation processing has been given between frames of the input video signal, an active-matrix liquid crystal display panel 104 having a liquid crystal layer and an electrode for applying the scan signal and the data signal to the liquid crystal layer, and an electrode driving portion 103 for driving a scan electrode and a data electrode of the liquid crystal display panel 104 based on the image signal subjected to the frame rate conversion by the FRC portion 100.
The FRC portion 100 includes a motion vector detecting portion 101 that detects motion vector information from the input image signal and an interpolation frame generating portion 102 that generates interpolation frames based on the motion vector information acquired by the motion vector detecting portion 101.
In the above configuration, for example, the motion vector detecting portion 101 may obtain the motion vector information with the use of a block matching method and a gradient method described later or if the motion vector information is included in the input image signal in some form, this information may be utilized. For example, the image data compression-encoded with the use of the MPEG format includes motion vector information of a moving image calculated at the time of encoding, and this motion vector information may be acquired.
FIG. 2 is a diagram for explaining a frame rate conversion processing by the conventional FRC drive display circuit shown in FIG. 1. The FRC portion 100 generates interpolation frames (gray-colored images in FIG. 2) between frames with the motion compensation using the motion vector information output from the motion vector detecting portion 101 and sequentially outputs the generated interpolation signals along with the input frame signals to perform processing of converting the frame rate of the input image signal from 60 frames per second (60 Hz) to 120 frames per second (120 Hz).
FIG. 3 is a diagram for explaining an interpolation frame generation processing of the motion vector detecting portion 101 and the interpolation frame generating portion 102. The motion vector detecting portion 101 uses the gradient method to detect a motion vector 111 from, for example, a frame #1 and a frame #2 shown in FIG. 2. The motion vector detecting portion 101 obtains the motion vector 111 by measuring a direction and an amount of movement in 1/60 of a second between the frame #1 and the frame #2. The interpolation frame generating portion 102 then uses the obtained motion vector 111 to allocate an interpolation vector 112 between the frame #1 and the frame #2. An interpolation frame 113 is generated by moving an object (in this case, an automobile) from a position of the frame #1 to a position after 1/120 of a second based on the interpolation vector 112.
By performing the motion-compensated frame interpolation processing with the use of the motion vector information to increase a display frame frequency in this way, the display state of the LCD (the hold display mode) can be made closer to the display state of the CRT (the impulse display mode) and the image quality deterioration can be improved which is due to the motion blur generated when displaying a moving image.
In the motion-compensated frame interpolation processing, it is essential to detect the motion vectors for performing the motion compensation. Proposed as the motion vector detecting method are, for example, the pattern matching methods described in “a method for detecting motion in a television image” of Japanese Laid-Open Patent Publication No. 55-162683 and “a method for asymptotically detecting an image motion vector” of Japanese Laid-Open Patent Publication No. 55-162684, or the iterative gradient methods described in “an image movement amount detecting mode” of Japanese Laid-Open Patent Publication No. 60-158786 or “an initial deflection mode in estimating motion of a moving image” of Japanese Laid-Open Patent Publication No. 62-206980.
Especially, the motion vector detecting mode according to the latter iterative gradient method is smaller and may detect the motion vectors more accurately compared to the pattern matching method. That is, in the motion vector detecting method according to the iterative gradient method, each frame of digitalized television signals is divided into blocks of a previously given size, for example, with m by n pixels including m pixels in the horizontal direction and n lines in the vertical direction and the calculations of the iterative gradient method are applied for each block based on the signal gradient in the screen and a physical reaction of a signal differential value between corresponding screens to estimate a movement amount.
Meanwhile, a moving image has a high correlation between frames and continuity in time scale. A pixel or block moving in a certain frame also moves with a similar movement amount in the subsequent frames or previous frames in many cases. For example, in the case of a moving image in which a ball rolling from right to left in the screen is shot, an area of the ball moves with the similar movement amount in any of the frames. That is, a motion vector often has continuity between consecutive frames.
Therefore, by referring to a result of the motion vector detection in the previous frame, the motion vector detection in the subsequent frame may be performed more easily or more accurately. The Japanese Laid-Open Patent Publication No. 62-206980 proposes a method that as an initial value for estimating a movement amount, from the motion vector candidates that have been already detected in a plurality of surrounding blocks including a block corresponding to the detected block, an optimum one for the motion vector detection in the detected block is selected as an initial displacement vector and the calculation of the gradient method is started from the value close to a true motion vector in the detected block so that the number of the calculations of the gradient method is reduced to detect the true motion vector, for example, by performing the calculation of the gradient method twice.
FIG. 4 shows an example of a motion vector detecting portion that performs vector detection with reference to a result of the motion vector detection in the previous frame. This explains an example of an internal configuration of the motion vector detecting portion 101 included in the FRC portion 100 of the image displaying device shown in FIG. 1 in detail. The vector detecting portion 101 has a frame delaying portion 1, an initial displacement vector selecting portion 2, a motion vector calculating portion 3, a vector memory 4 and a zero-vector holding portion 5, and acquires, for each of the detected blocks, a motion vector that represents a direction and a size of the motion between corresponding blocks, for example, of an input image signal of the immediately previous frame that has been delayed by the frame delaying portion 1 and an input image signal of the current frame.
The initial displacement vector selecting portion 2 selects an optimum motion vector as an initial displacement vector in the detected block by using a candidate vector group selected from motion vectors that have been already detected and accumulated in the vector memory 4 and a length-zero-vector (hereinafter referred to as zero-vector) supplied from the zero-vector holding portion 5. The motion vector calculating portion 3 uses the selected initial displacement vector as a starting point to acquire the true motion vector in the detected block, for example, by performing the calculation of the iterative gradient method twice.
Furthermore, the true motion vector calculated by the motion vector calculating portion 3 is stored in the vector memory 4 and is used as a candidate of the initial displacement vector for the motion detection processing in the next frame. When a moving object moves through and a background appears in a certain area of the screen, the movement amount changes rapidly from a certain level to zero, but this change may also be followed by adding zero-vector to the candidates of the initial displacement vector. This method is disclosed, for example, in Japanese Laid-Open Patent Publication No. 2005-301622.
Furthermore, Japanese Laid-Open Patent Publication No. 06-217266 discloses “a motion vector detection circuit” in which a method for detecting an initial displacement vector of motion between blocks of image signals that are away from at least one or more field or frame each other is proposed to further improve accuracy of the motion vector detection. Also in the block matching method, efficient motion vector detection is thought to be performed by changing a search order with reference to the result of the motion vector detection in the previous frame. In this way, by using the already-detected motion vector for detecting the motion vector, for example, a real time processing of the frame rate conversion is possible.